You can implement a unified content strategy without XML, using traditional
authoring tools, but XML provides you with the ability to do a whole lot more.
There are disadvantages to XML, notably that it is a new technology that brings
issues in dealing with the learning curve, the complexity, and the
implementation. However, the disadvantages are outweighed by the advantages. The
characteristics of XML that best support reuse are:

Structured content

Separation of content and format

Built-in metadata

Database orientation

XSL style sheets

Personalization

These are described in more detail in the following sections.

XML and structured content

Authors typically have a high-level understanding of the concept of
structured content. For example, they understand that books have front matter,
body chapters, and back matter. Authors may also recognize repeatable structures
at a lower level. Chapters have titles, overviews, sections containing the
"meat" of the chapter, and a summary. Some authors can even describe
the structure in individual sections, for example, a procedure. However, when
you examine similar information products, you find that structures are not
consistent from product to product. Structures will vary from author to author,
from department to department, from division to division. Even information
written by a single author will vary over time. This is a big problem for
reuse.

In XML, structure can be defined in a Document Type Definition (DTD) or
Schema1. A DTD is quite specific; it defines
all the elements (XML tags) that can be used in a document. It also defines the
relationship of those elements to other elements. You can specify the hierarchy
of elements ("a chapter contains..."), the order of elements, or even
the number of elements.

A DTD can be incredibly valuable for the writing process. Many authors take
as much time figuring out the structure they need to write to as they do
actually crafting the information. Does my presentation need an overview? Does
my procedure have an introduction? Do I need to include a title for a graphic?
With a DTD, you can mandate the structure that is required. This consistency is
also very valuable for the information's users. Consistency leads to
predictability. Users learn where information is to be found and can
automatically navigate to it, finding what they need quickly and efficiently. In
addition, a DTD provides a powerful map for systematic reuse and
personalization. When content is systematically reused, the content management
system must identify what content can be reused where. The DTD specifies this
information. Personalization also requires a map and set of rules to define what
information should be provided and in what order. The DTD provides this
information.

For structural consistency, having a defined structure in a DTD is half of
the solution. The other half is provided by specialized editing tools (called
validating editors) that can read a DTD and enforce the structural rules
defined in it. By providing authors with a validating editor and a DTD, you can
ensure that all your information products are structurally consistent.